I'd say when you get bored, try a new install on the Marvell controller the way I mention above with NO Marvell driver on your SATA 6Gb/s disk
OK Lsdmeasap, tried your method this morning with the result being a complete failure. The Western Digital SATA3 6GB/s and the Marvell 9128 SATA3 controller on the GA-790XTA-UD4 when set to AHCI do not play happily.
I installed Windows 7 64 Bit without an issue. I didn't install any other drivers at this point. I then installed CrystalDisk to run a speed test, the system crashed.
Rebooted the PC loaded the AMD Northbridge driver, tried to run CrystalDisk, again and the system crashed.
Rebooted, check Device Manager and there was an exclamation mark on the Marvell 91XX installed Marvell Driver, rebooted tried running CrystalDisk system crashed.
Rebooted, tried to open web browser, system crashed.
Spent 4 hours trying various re-install/driver combinations with AHCI enabled and system would either hang or crash as the mood would take it and would not run for longer than 5 minutes without doing so.
CONCLUSION: Western Digital Caviar Black 640Gb SATA3 6GB/s Mechanical Hard drive and Marvell 9128 SATA3 do not work in AHCI mode.